Product and process

ABSTRACT

High-strength fiber is prepared by coating a small grain, small diameter polycrystalline refractory oxide fiber with a specified amount of a glass-forming oxide or its precursor, and heating the coated fiber at a temperature sufficient to decompose the precursor to the oxide and/or to vitrify the oxide coating into an adherent, optically uniform, thin layer. The coating significantly increases the tensile strength of the fiber, while the fiber retains a substantial percentage of its original modulus. The fibers are especially suitable for reinforcement uses.

Unite ll Green States Patent [451 Nov. 19, 1974 I PRODUCT AND PROCESS[75] Inventor: James Ralph Green, Wilmington,

Del.

[73] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

22 Filed: June 30,1972

21 Appl. No.: 268,024

Related US. Application Data [63] Continuation-impart of Ser. No.35,064, May 6,

1970, abandoned.

[52] US. Cl 117/106 R, 117/46 FA, 117/46 FC, 117/169 R, 117/169 A [51]Int. Cl C236 13/04 [58] Field of Search 117/106 R, 106 A, 100 B,117/121, 169 R, 169 A, 46 FA, 46 FC [56] References Cited UNITED STATESPATENTS 3,131,087 4/1964 Paquet 117/169 3,207,699 9/1965 Harding et al.117/100 X 3,212,921 10/1965 Pliskin et al. 117/101 3,227,032 l/l966Upton 88/1 3,322,865 5/1967 Blaze 23/17 3,416,953 12/1968 Gutzeit et al.117/169 X 3,712,830 1/1973 Kirchner ll7/169 R X FOREIGN PATENTS ORAPPLICATIONS 1,126,135 Great Britain OTHER PUBLICATIONS AD 649,537, HighTemperature Fibers and Core-- Sheath Fiber Development," cover page, i,iv-v, 1,1415,1819,24-27.

AD 438,145, Silica Fiber Forming and Core-Sheath Composite FiberDevelopment, cover page, title page, ii, iii, 35,42,55 59.

Primary ExaminerLeon D. Rosdol Assistant Examinerl'larris A. Pitlick[57] ABSTRACT High-strength fiber is prepared by coating a small grain,small diameter polycrystalline refractory oxide fiber with a specifiedamount of a glass-forming oxide or its precursor, and heating the coatedfiber at a temperature sufficient to decompose the precursor to theoxide and/or to vitrify the oxide coating into an adherent, opticallyuniform, thin layer. The coating significantly increases the tensilestrength of the fiber, while the fiber retains a substantial percentageof its original modulus. The fibers are especially suitable forreinforcement uses.

16 Claims, 2 Drawing Figures PATENTE-u 1 91974 3, 849 .181

SHEET 2 BF 2 FIG. 2

PRODUCT AND PROCESS CROSS REFERENCE TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of application Ser. No. 35,064,filed May 6, 1970 and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to high-stength refractory fibers and to methods for preparingthem.

2. Description of the Prior Art Polycrystalline refractory oxide fibers,particularly alumina, of long lengths are very desirable due to thecombination of high theoretical tensile strength and modulus withchemical inertness at ambient temperature and the retention of asubstantial portion of these properties at temperatures above 1,000C.Many efforts have been made to produce such fibers as shown for examplein US. Pat. No. 3,311,689 to Kelsey and US. Pat. No. 3,327,865 to Blaze.However, previous fibers have only had a small fraction of thetheoretical strength.

It has been proposed in publication AD 649,537 by the US. Department ofCommerce to simultaneously extrude a polycrystalline oxide core and aglass sheath. This method has yielded small amounts of fibers withincreased tensile strength but with low modulus.

SUMMARY OF THE INVENTION The present invention provides high-strengthfiber comprising a polycrystalline refractory oxide fiber having a fiberdiameter between about 3 and 250 microns and comprised of grains havinga median grain diameter of 1) less than about 3 microns and (2) lessthan about percent of the diameter of the fiber, which has adhered to ita vitrified coating of a glass-forming composition comprising aglass-forming oxide, in the form of an optically uniform layer, theapparent thickness of the layer being (l) less than about 1 micron and(2) less than about 5 percent of the diameter of said fiber core.

The present invention also provides a process for preparing the highstrength fiber described in the preceding paragraph which comprisesapplying a coating composition comprised of molecular to colloidal sizeparticles of a glass-forming oxide or its precursor to the surface of apolycrystalline refractory oxide fiber also described in the precedingparagraph, said composition being applied in an amount sufficient toprovide a vitrified layer (which results from the subsequent heatingstep) having an apparent thickness of less than about 1 micron and lessthan about 5 percent of the fiber diameter; followed by heating thecoated fiber at a temperature and for a time sufficient to vitrify thecoating into an adherent, optically uniform layer.

DESCRIPTION OF THE DRAWINGS The invention may be better understood byreference to the accompanying drawings in which:

FIG. 1 illustrates a coated fiber of this invention; and

FIG. 2 is a series of photomicrographs illustrating polycrystallinerefractory fibers before and after coating.

DESCRIPTION OF THE INVENTION The term polycrystalline is used to meanthat the fiber comprises numerous refractory oxide crystals rather thana single crystal. The term refractory oxide is used to mean an oxidethat melts at least as high as 1,000C. Such refractory oxides include A10 MgO. ThO ZrO Cr O Fe O NiO, CoO, Ce O 0 U0 BeO, HfO TiO La O andmixtures of oxides Such as 3AI203'2SI02, AI2O3'AIPO4, 21 02" SiO ZrO-l-CaO, and ZrO +MgO.

A preferred fiber contains at least 60 percent by weight of a singlesimple refractory oxide, which is most preferably alumina. The remaining0 to 40 percent comprises other refractory oxides which may be presentas separate phases, as part of a compound or as a solid solution withanother refractory oxide. Other oxides which are not consideredrefractory oxides, such as B 0 P 0 A5 0 TeO and SiO may be presentwithin the fiber in amounts that do not reduce the melting point of thefinal fiber below 1,000C. Most preferably the preferred alumina fiberswill contain only up to about 5 percent of one or more of the oxides ofcobalt, magnesium, lanthanum, nickel, copper or cadmium.

The fibers are comprised of grains having a median grain diameter ofless than about 3 microns and less than about 10 percent of the fiberdiameter. Grains within this definition are believed to provide thefiber with a high degree of internal strength which is necessary toprovide the maximum strength advantage of the fibers used in thisinvention.

Preferably, the porosity of the fibers is below 20 percent, mostpreferably below 10 percent. These low porosity fibers are preferred foruse herein because they can be strengthened to a higher absolute levelof strength when coated in accordance with this invention than fiberswith a higher degree of porosity. Fibers which yield higher absolutestrengths appear to have a relatively smaller number of internal flaws(pores and weak grain boundaries) as indicated by the amount oftransgranular cleavage occurring upon fracturing. In contrast, fibershaving lower strengths tend to cleave along grain boundaries whenfractured (indicating more pores and more weak grain boundaries).

A generally preferred fiber for use in this invention I has a porosityless than 10 percent, a crystallinity greater than percent by weight anda grainsize distribution wherein substantially none of the grains arelarger than about 3 microns and at least 30 percent by weight aresmaller than about 0.5 micron.

The fibers will have a diameter between about 3 and 250 microns, andpreferably, because of ease of preparation, between about 5 and microns.Most preferably the diameter will be between 6 and 50 microns. TheVitrified Coating The term vitrified coating" as used herein means acoating that has been heated to melting or sintering and then cooled toform a glass-like coating.

The vitrified coating may be formed from a coating composition whichcontains an oxide, or a precursor of the oxide, as well as intermediatesand modifiers and their precursors.

The term glass-forming oxide is used to mean any of the oxides, alone orin combination with other oxides, which can form a glass upon coolingfrom the liquid state. Suitable oxides include those which form glassesby themselves, i.e., glass formers (especially the oxides of silicon,boron, germanium and phosphorus), and combinations of one or more glassformers with one or more oxides known as intermediates and/or modifiers(see Introduction to Ceramics by W. R. Kingery, John Wiley and Sons, NewYork 1960, Chapter 5, and especially Table 5.1 listing glass formers,intermediates and modifiers). Preferably the oxides should have amelting point above 800C.

An intermediate is a compound which will not form a glass by itself, butis capable of being incorporated into the atomic networks thatcharacterize polycomponent glasses. A modifier is similar to anintermediate except that it does not form part of the network, butrather is believed to occupy interstices which occur in the atomicnetwork. Suitable intermediates which can be used include: titaniumdioxide, zinc oxide, lead oxide (PbO), and beryllium oxide. Suitablemodifiers which can be used include: lithium oxide, magnesium oxide,calcium oxide, cadmium oxide, barium oxide, and strontium oxide.

The precursors of the oxides (i.e., materials which are converted to theoxides) include, for example, silicon tetrachloride (pure or partiallyhydrolyzed) or organosilicon compounds whichare convertible to silica.Among the other suitable precursors may be named boron trichloride,boron tribromide, phosphorus trichloride, phosphorus oxychloride,germanium tetrachloride, and similar arsenic compounds. The particularprecursor used should be selected in view of the ease of handling, theboiling point and the amount of the vitrified coating desired.

Coating compositions which contain at least about 50 percent precursorsthat will provide silica, are preferred, but even more preferred arethose that will provide a vitrified coating that is substantially allsilica. Application of the Coating Composition to the Fiber The coatingcomposition may be in solid, liquid or vapor form when applied, andshould be comprised of molecular to colloidal size particles. Since amajority of the suitable oxides, and any accompanying intermediates andmodifiers that may be present are insoluble in water, they are readilyapplied as an aqueous dispersion of colloidal size particles of theoxide itself. For example, silicon dioxide, aluminum oxide, titaniumdioxide, stannic oxide, germanium dioxide, zirconium dioxide, magnesiumoxide and lead oxide (PbO) form relatively stable dispersions undersuitable conditions (e.g., concentration, temperature and particlesize).

Precursors of the oxides are generally more conveniently applied to thefiber as a liquid or vapor. This method of application is highlypreferred in that bundles (e.g., closely aligned continuous filamenttow) of uncoated fibers can be coated without the fibers sticking to oneanother (after coating or after firing). This lack of sticking issurprising since, generally, aqueous colloidal dispersion coatingtechniques are only suitable for monofilaments (i.e., the fibers must bekept separated to prevent sticking). A useful method for coating fiberswith a water-reactive precursor of silica such as silicon tetrachloridecomprises exposing the fibers to an atmosphere, e.g., steam, wherein therelative humidity is greater than about 50 percent. The moist fibers arethen passed through the liquid or vaporized silicon tetrachloride whichreacts with water on the fiber surface to form a uniform layer ofhydrated silica.

A preferred method for coating the fibers comprises passing the uncoatedfiber, immediately after firing, into a bath that contains a solution ora dispersion of the precursor.

The coating composition may be applied to short (staple) fibers or tocontinuous lengths, individually or in groups (e.g., yarns or slivers)as described above.

A sufficient amount of the coating composition is applied to provide avitrified layer having an apparent thickness of between about 0.01micron and about 1 micron and less than about 5 percent of the uncoatedfiber diameter. The actual amount of material applied depends on theform (vapor, liquid, solution, solid), concentration, and composition ofthe coating composition as well as the number of coating cycles.

It is preferred that relatively small amounts of the coating compositionbe applied to the fiber, i.e., just enough to provide a thin uniformcoating.

Heating of the Coated Fiber The oxides or precursors that have beenapplied using the methods set forth in the preceding section arevitrified by heating the fiber at a temperature above the melting orsintering temperature (e.g., greater than 1,l0OC. for SiO which issufficient to vitrify the coating into an adherent, optically uniformlayer. The term vitrify is used herein to mean that the heatingconditions are sufficient to cause sintering or melting of the oxide.The particular temperature and time sufiicient to vitrify a givencoating composition may be selected from reasonably broad ranges withshorter times being satisfactory at higher temperatures. For example,amorphous silica may be vitrified when heated at a temperature of about1,100C. for many hours, when heated at a temperature of about 1,350C.for about 30 seconds or when heated at a temperature of about 1,500C.for about 5 seconds. Similar ranges exist for vitrifying the otheroxides and are easily determinable. A preferred method for heatngsilica-coated fiber comprises passing the coated fiber through the flameof a propane-air torch (generally 1,500C. 1,900C., depending on thepropane/air ratio) for a residence time in the flame of about 0.1 to 5.0seconds.

During the heating process, in addition to vitrification of the coatingmaterial, precursors are converted to their respective oxides, and thewater of hydration (of hydrated materials, if any) is driven off to forma substantially anhydrous coating. Additionally, any carrier liquid isvolatilized and some vol'atile precursor may be vaporized.

It has been observed that if the heating step is omitted, the coatingmaterial is not vitrified as described above and the resultant fibersexhibit substantially no increase in tensile strength when compared withuncoated controls. Even if the coating material consisted of vitrifiedparticles, vitrification in situ is still believed to be necessary toprovide the necessary adherence (discussed hereinafter) of the coating.

Although a variety of combinations of heating times and temperatures (asdescribed above) may be used in the heating step, prolonged exposure orexposure at excessive temperatures has been observed to result in a lossof strength. This strength loss may be due to either a loss of thecoating by volatilization or to diffusion of the coating into the fibersubstrate.

The Coated Fiber Product of the Invention The fiber coating is thevitrified oxide already discussed. A relatively thin layer of thecoating (i.e., less than about 1 micron in apparent thickness,preferably less than about 0.1 micron), which is also less than about 5percent of the fiber diameter, provides significant improvements intensile properties. Because it is difficult to directly measure thethickness of these thin layers on the small diameter fiber substrates,the apparent thickness may be calculated from the amount of coatingmaterial on a large group of fibers and the density of the coatingmaterial as described later.

It has been observed in a series of coatings on the same fiber substratethat a significant increase in tensile strength in coated fibers overuncoated fiber substrates is obtained when the coating is at least about0.01 micron thick and that the tensile strength increases as the coatingthickness increases up to about 0.1 micron, after which the tensilestrength decreases.

The preferred products of this invention will have a tensile strength ofat least 100,000 pounds/square inch (psi) and more preferably at least200,000 psi (i.e., 7,000 and 14,000 kilogram/cm respectively). Preferredembodiments of this invention are also characterized by an elasticmodulus (fiexural) of at least 40,000,000 psi (2,800,000 kg/cm Althoughthe invention is not to be limited by the theoretical explanationthereof, it is believed that the coating heals small surface defects inthe fiber surface and it is those portions of the fiber surface thatmust be coated. The required type of coating is assured by the thinoptically uniform layer of material as described herein. It is furthertheorized that the surface defects result, at least in part, from theimperfect alignment of grains in the polycrystalline fiber at the fibersurface which creates asperities. It is therefore believed that to healthese defects, an apparent coating thickness of less than l the mediangrain diameter is desirable to fill or partially fill the asperities.When attempts are made to use larger amounts of coating, at least twoproblems arise. Firstly, spalling occurs, i.e., a degree or zone ofcoating thickness is reached whereupon the coating is no longeroptically uniform due in part to its inability to withstand stresses(e.g., due to a differential thennal expansion) and the coating breaksaway from the core. Secondly, if relatively thick coating (beyond thezone of spalling) is applied there is a sacrifice in the desirably highmodulus of the refractory oxide fiber substrate due to the lessermodulus of the coating (i.e., as the volume or thickness of therelatively low modulus coating increases, the modulus of the coatedfiber decreases). It is therefore believed that both the less than about1 micron and the less than about 5 percent of the fiber diametercharacterizations of the apparent coating thickness are importantherein; the former assures that the required coating uniformity toprovide high tensile strength is obtained, while the latter restrictsthe volume of coating to approximately the maximum required tocompletely fill asperities (since the median grain size is less than 10percent of fiber diameter) and assures that the coated fiber retains thedesired high modulus (the maximum volume of coating based on thisrestriction is l7.4percent).

The coating must adhere to the fiber substrate as described above, i.e.,the coating must be optically uniform after being subjected to thecleaning procedures described hereinafter. The in situ vitrification(described hereinbefore) provides the desired adherence. It is believedthat the coating layer is bonded to the fiber substrate through aninterface (a product of a reaction between the coating and the fibersubstrate). However, the interface is generally difficult to detectbecause it is so small.

FIG. 1 illustrates a coated fiber I of this invention. The coating 2 isin the form of an optically uniform layer of the required thicknessadhered to the surface of the substrate 3. The coating 2 is not ofconstant thickness as illustrated (although it may be) and may or maynot have small uncoated portions 4 which generally may appear atelevated points (generally protruding grains) on the fiber surface. Thefiber segment which is uncoated illustrates the grains 5 of which thefiber is comprised.

Utility The high strength (i.e., high tensile strength and high modulus)fibers of this invention are particularly useful as reinforcing agentsfor plastics, metals, ceramics and other materials. These fibers may besubstituted for uncoated refractory fibers in various end-uses.especially where high tensile strength and high modulus are desired,e.g., filament-wound radomes and sonardomes, high temperature jet-enginevanes and support structures, and truss members in air frames.

A useful segment of a given fiber must be coated as described herein,i.e., the portion of the fiber that is stressed in use should be coated.If the particular enduse requires that the fiber be of uniform strengthalong its entire length, then the entire fiber surface should have thecoating described above. On the other hand, if only segments of thefiber are subjected to stress in use, then only those portions need becoated.

For some applications such as filament-wound reinforced plastics andweaving, it may be advantageous to apply a sizing (e.g. starch) or afinish (e. g. a solution of gamma-amino propyl triethoxy silane) thatwill adhere to the filaments and be compatible with resins appliedsubsequently.

MEASUREMENT AND TESTING PROCEDURES Presence and Uniformity of CoatingMethod a A substantially straight 2.5 cm.-long fiber sample is placed ina liquid exhibiting a refractive index which matches the refractiveindex of the substrate fiber (e. g., 1.760 for alpha alumina). Thesample is examined using plane polarized white light, and a microscopewith reasonably high numerical aperture optics (NA of about 0.85) and amagnification power of 600X. The fiber is positioned such that thelongitudinal axis is substantially parallel with respect to the plane ofpolarized light. The image of the fiber-liquid interface is criticallyfocused to obtain optimum resolution. The observation of a line that issubstantially parallel with respect to the fiber axis and coextensivewith the fiber-liquid interface indicates the presence of a coating onthe fiber.

Observation of a substantially continuous line, indicates the coating isoptically uniform to the degree that is considered necessary for theresults of this invention, notwithstanding the fact that small portionsof the fiber surface may be uncoated and/or that the coating may not beof constant thickness. If the sample does not show an optically uniformcoating by this method it should be examined by method b.

Method b A single fiber is mounted on a microscope slide and the fiberimmersed in a liquid that matches the refractive index of the substratefiber. The fiber and liquid are covered with a cover glass. The fiber isviewed at -l,000X( lOOX objective and 10X eyepiece) in oil immersion(cedar oil placed between the cover glass and objective to optimizeresolution) on a Phase Contrast Microscope.

Briefly, a Phase Contrast Microscope converts optical path difference,which is the product of (thickness) and (index of refraction variation),into an intensity difference in black and white which is discernible bythe eye as contrast in the image. Since the immersion medium matches thesubstrate fiber, contrast in the image is due to variations in the indexof refraction of the areas exhibiting the contrast.

The fiber is scanned along its length in phase contrast and has anoptically uniform coating if a random area exhibits continuous phasecontrast along both edges in the entire field of view (approximately 0.1mm at 1,000X). This method 12 is more sensitive and precise than methoda and is preferably used.

Composition of the Vitrified Coating The composition of the coating isdetermined by dissolving the coating material and analyzing the solutionusing conventional chemical analysis methods for the various elements.

Quantity of the Vitrified Coating The amount of coating material presenton the fiber substrate is determined by removal of the coating from a0.5 to 1.0 gram fiber sample using a suitable etching agent that willdissolve the coating without substan tially affecting the fibersubstrate. For example, a 48 percent aqueous solution of hydrofluoricacid has been found satisfactory as an etching agent for silica coatedalumina fibers. When etching is complete, any excess etching agent isremoved by heating the sample to 900C. This etching process is repeateduntil no weight difference is apparent following successive treatments.A weight correction equivalent is added to the observed weight loss tocompensate for a weight change which has been observed when uncoatedfibers are treated with the etching agent. For example, for the silicacoated alumina fiber of Example 9, a correction of 0.03 percent is addedto the observed weight loss.

As an example of another method, fibers of Examples 2, 3, 4 and 10 arefused with sodium carbonate; the melt dissolved in HCl and the solutiondiluted to a known volume. The concentration of silicon in the solutionis obtained by using an Atomic Absorption Spectrophotometer (Model 303by Perkin-Elmer Corp. of Norwalk, Conn.) and the weight of the coatingcalculated. See Analytical Methods For Atomic AbsorptionSpectrophotometry published by Perkin-Elmer, Norwalk, Conn., 1971.

Apparent Coating Thickness The apparent coating thickness (u, inmicrons) for a fiber of round cross section is calculated from theamount of coating material per square meter of fiber substrate surfacearea and the density (d in g/cc of the coating material using thefollowing equation:

The density (d is determined by conventional means (a value of 2.19 g/ccis used for silica).

The quantity g/m is calculated using the equation:

g/m DW/4V The fiber substrate diameter (D), expressed in meters. ismeasured using a microscope equipped with a filar micrometer eyepiece. Vand W represent the volume (in cubic meters) and weight (in grams),respectively. of the fiber substrate sample.

The apparent coating thickness for nonround fibers can be calculated inan analogous manner using photomicrographs of the coated fiber or thesubstrate fiber to obtain the dimension of the substrate fiber.Characteristics of the Coating The vitrified nature of the coating isverified by testing the solubility of the coating in a liquid which isknown to be a solvent for the coating in nonvitrified form. If thecoating is vitrified, it will be substantially unaffected (as determinedby the optical procedures previously described) under conditions whichwould dissolve the nonvitrified material. For example, if silica is thecoating material, nonvitrified silica is removed during a two-hourimmersion of the fiber in a 20 percent aqueous solution of sodiumhydroxide at ambient temperature. Over the same period of time vitrifiedsilica is substantially unaffected by this reagent.

The adherence of the coating to the substrate is verified by subjectingfiber having a substantially uniform coating (as verified by the opticalprocedures previously described) to a cleaning in a 0.1 percent aqueousconventional detergent (e.g., Tide) solution for 10 minutes at 50C. withmild manual stirring. The fibers are rinsed and dried and thenreexamined by the same optical procedures to determine whether thecoating is still present.

Characteristics of the Fiber Substrate Porosity of the fiber iscalculated using the following equation:

% Porosity (Apparent Density Bulk Density/Apparent Density) X 100.

The apparent density is obtained using an air pycnometer and a samplesize of about 0.1 g. Prior to being evaluated the fiber is fired for 2minutes at 1,500C. The fiber is then pulverized using a mortar andpestle to produce lengths that are no more than five times the averagefiber diameter thereby minimizing any closed void content in order toobtain an apparent density value which closely approximates or equalsthe true density of the sample.

The bulk density is the weight of fiber divided by the (area ofcross-section X fiber length). Fibers are straightened in a propane-airflame for bulk density measurements in order that fiber length caneasily be measured. Fiber lengths are measured using a microscopeequipped with a micrometer and noting the displacement required to scanthe entire length of the sample. The diameter of round fibers ismeasured with a precision of 2.5 X 10 mm. using a microscope fitted witha filar eyepiece. The area of noncircular crosssections is measuredusing photographs of fiber ends. Fibers are weighed on a balance capableof weighing accurately to l X. 10' gm. using a minimum sample of 1 X 10gm.

The percent crystallinity of the fiber may be determined using thetechnique described by H. P. Klug and L. E. Alexander in X-RayDiffraction Procedures for Polycrystalline and Amorphous Materials, pp.626-633, published by John Wiley & Sons, Inc., 1954. A suitablemodification of this technique which is used to determine the amount ofoz-alumina present in preferred fibers of the invention is as follows(this procedure, with proper calibration, is applicable to all fibers ofthis invention):

A calibration curve for percent crystallinity versus X-ray intensity isobtained as described below.

Mixtures of a-alumina (100 percent crystalline) and glass percentcrystalline), both passing 325 mesh, are prepared containing and percentof the glass and homogenized using a mortar and pestle. The X-rayintensity for these mixtures and for 100 percent a-alumina is determinedon an X-ray diffractometer equipped with a wide range goniometer, copperKa radiation, a nickel B filter, /2 divergent and scatter slits,scintillation detector, and pulse height analyzer. The total amount(i.e., integrated) of diffracted intensity (1,) from l2.00 to 45.33 (20)and the intensity (1 from 37.00 to 40.33 (20) is obtained using standardcounting procedures as the sample is rotated at a rate of 2 (26) perminute, all analyses being carried out in duplicate. The intensity ratio21 is then calculated and plotted versus the percent crystallinematerial in the sample; the best straight line is drawn through the datapoints.

The same intensity ratio is measured for each of the fiber samples afterthey are ground to pass a 325mesh screen and the percent crystallinityis then obtained from the previously determined calibration curve. Thealumina fibers used as substrates in the examples have a percentcrystallinity of 85 to 100 percent.

The grain size and size distribution on the longitudinal surface of thefibers is determined from an enlarged electron micrograph following anextension of the method of John E. Hilliard described in Metal Progress,May 1964, pp. 99-102, and of R. L. Fullman, described in the Journal ofMetals, March 1953, p. 447 and ff.

An etch will be necessary to remove the coating and reveal the grainsbut should not substantially affect the grains themselves. As anexample, alumina fibers coated with silica may be etched for 30 minutesin concentrated (48 percent) hydrofluoric acid at room temperature.Standard electron microscope procedure is used to obtain electronmicrographs. Carbon is deposited directly on the platinum-shadowedetched (or unetched) fibers. The fibers are completely dissolved (hotphosphoric acid at about 350C. being used for alumina fibers) from thecarbon replica which is washed and examined on the electron microscope.A representative area is photographed at about 2,500 fold magnification.The negative is then enlarged to produce a photomicrograph that exhibits20,000 fold magnification.

Three or four circles each having a radius of 6.4 centimeters, are drawnin different areas of the enlarged micrograph so that a total of atleast 100 grains will be intersected by the circumferences of all thecircles. The intersections of the circumference with each grain boundaryintersecting the circumference are marked on all circles.

The length of the chord corresponding to the arc indicated on thecircles for each of the grain intersections is measured and the measuredlengths are tabulated in the following fractions: l-2 millimeters, 2-4mm., 4-8 mm., 8-16 mm., 16-32 mm., and 32-64 mm.

The average chord length, d for each of the size fractions can becalculated by dividing the sum of the chord lengths for the sizefraction by the number of grains measured in the size fraction andconverting to actual dimensions in the sample in angstroms. This isconverted to average grain diameter, d by the formula of Fullman:

un 77/2 (m).

The average grain diameter and the percent of grains in each sizefraction for a typical alumina fiber used in the examples follows: 0.16p. (2%), 0.31 (l 1%), 0.47 (51%), 0.86 (34%), 1.57 (3%).

The size distribution data are plotted as cumulative percent vs. averagegrain diameter using log-normal probability paper (probability andlogarithmic scales, the former based on the normal law of error). Thebest straight line is fitted to the data points between 10 and 98percent. The average grain diameter corresponding to 50 cumulativepercent on this line is the median grain diameter. A typicalcoated-alumina fiber of the examples has a median grain diameter of 0.43(from above distribution).

Fiber Tensile Properties Tensile strengths are measured at ambient roomconditions using a method by R. D. Schile et al. in Review of ScientificInstruments, 38 No. 8, August 1967, pp. 1l034. The gauge length is 0.04inch (0.1 cm.) and the crosshead speed is l-4 mils/min.

Elastic moduli (flexural modulus) are measured by vibroscope techniquesas described in J. Applied Physics, Vol. 26, No. 7, 786, 792, July,1955.

Preparation of Refractory Oxide Fibers Used as Substrates A preferredmethod as described in Offenlegungsschrift 1,913,663 of September, 1970,to Seufert, utilizes a two-phase spinning mix containing small particlesof a refractory oxide such as alumina, zirconia etc. in an aqueoussolution of a salt convertible to a refractory oxide upon heating(termed a precursor of a refractory oxide). Such spinning mixes may beconcentrated and/or aged by heating (e. g., about C.) to improve theability of the spinning mix to be extruded and to aid in attenuating theextruded fiber. The spin mix is extruded through orifices and theextruded fiber attenuated to form as-spun fiber. The as-spun fiber isgenerally fired in two stages. The first or low temperature stage (e.g.,heat slowly to 500 to 900C.) removes the water and other volatile matterand may partially or completely decompose the precursor. The second orhigh temperature firing (e.g., l,300 to 1,500C.) results in theformation of oxides, sintering of the oxide grains, and development ofcrystallinity. Optionally a final flame firing straightens the fibersand results in further grain growth and reduction of porosity. Thefibers of Examples 1 to 9 are made by this technique.

US. Pat. No. 3,322,865 Blaze discloses the extrusion of viscous aqueoussolutions of mixed metallic salts followed by firing to refractory oxidefibers. This general method is used for the starting fibers of Examples10, 11 and 12.

Suitable fine particles of a-alumina (used in Examples l-9) are made byclassifying an aqueous dispersion (adjusted to a pH value of about 4.0with hydrochloric acid) containing about 20 percent of finely dividedaluminum oxide (XA-16, marketed by Aluminum Co. of America) bysedimentation to remove all particles larger than about 2 microns. Thedispersion is concentrated to about 40 to 70 percent aluminum oxide.Using the procedure of G. A. Loomis (J. Amer. Ceramics Society 21 393,1938) it is determined that about 100 percent of the particles in atypical classified product exhibit an equivalent spherical diameter lessthan 2 microns and about 89 percent exhibit a diameter less than 0.5micron.

THE EXAMPLES Parts and percentages in the following examples, as well asthroughout this patent, are by weight unless otherwise stated.

The conditions used to make the starting fibers, i.e., the substratefibers, for the examples are summarized in Table I. The ingredients ofthe spinning mix are mixed, well stirred, usually concentrated undervacuum, optionally aged at about 80C. and extruded through spinneretscontaining holes of about 0.05 mm. (Example 12), 0.1 mm. (all examplesexcept 8 and 12) and 0.2 mm (Example 8) diameter.

In some cases (Examples 2, 6, 7 and 11) a finish of a 20/80 volume ratioof ethyl laurate in perchlorethylene is applied to the fibers beforethey are wound on a bobbin.

Table I gives the calculated composition of the spinning mix afterconcentration, the weight loss of the original mixture upon beingconcentrated and the firing conditions for the extruded fibers.

Oxide particle A is the classified alumina previously described. Oxideparticle B are aluminum oxide-coated S102 particles (A1 13.5%; SiO2,86.5%; Positive Sol 130M sold by the Du Pont Co. of Wilmington,Delaware).

Codes used for oxide precursors are as follow:

Al-l aluminum chlorohydroxide dihydrate [Al (OI-1) Cl.2.2H O] from thesolid compound Al-2 Al (OH) Cl.2.21-1 O from a 50 percent aqueoussolution of the aluminum chlorohydroxide complex A1-3 basic aluminumacetate [A1(OH) (C H O from a 15.5 percent aqueous solution Al-4aluminum chloride from a 27.8 percent aqueous solution A1-5 basicaluminum acetate [AI (OH) (C H O from solid hydrate containing 85percent of this salt Cr-l hydrated chromium chlorohydroxide [Cr (OH C1.121-1 O] from the solid compound Ca-1 calcium acetate monohydrate fromthe solid compound Zr-l zirconium acetate [H ZrO (C 1-1 O from a 44percent aqueous solution In addition, some examples contain parts of HCl(calculated as 100% HCl) as follows: 1 (0.38), 2 (0.32), 3a (0.13), 6(0.4), 7 (0.38), 8 (0.43), and 9 (0.39). Example 11 also contains 0.25part of acetic acid. Example 9 contains 0.1 part of a polyethylene oxideto increase the viscosity.

The fiber substrates used in Examples 3 b to f are made in a similarmanner as 3a with the replacement of the cobalt salt as follows: b, NiC1.6H O (0.8 parts); 0, MgC1 .66H O (0.13 parts); d, CuC1 .2H O (0.56parts); e, La(NO .bH O (1.4 parts); and f, CdCl .2.5H O (1.2 parts) andfiring of 3c substrate under conditions T1 and D12; all others firedunder conditions T3, D2 and D3.

The concentrated spinning mixes of some examples contain small parts ofMgC1 .6l-1 O as follows: 6 (0.54 parts); 7 (0.55), 8 (0.63), 9 (0.35)and 10 1.6 parts).

Low temperature firing Tunnel furnace Fibers on a screen are carried ona belt through a tunnel furnace, 8 X 8 X 27-inch long interior (20 X 20X 69 cm) at a constant speed such that the fibers are in the maximumtemperature zone (6-inches long. 15 cm) for indicated time:

code maximum temperature. C. time. minutes Muffle furnace Fibers in aplatinum boat (M1 and M2) or on a refractory bobbin are heated in amuffle furnace as indicated M1 slowly heated from room temperature to550C.

where they are held for 45 minutes M2 heated to 540C. in one hour; thentemperature raised at a rate of 280C./hour to 820 to 870C. where it isheld for 5 to 10 minutes. M3 heated to 550C. and held for 45 minutes M4heated to 900C. over a period of 4 hours High temperature firingconditions Tube furnace Fibers in platinum boats are passed through a36-inch (92 cm) long tube furnace at a constant rate such that they areexposed in the 6-inch (15 cm) long zone, midway of the open tube, ofmaximum temperature for the indicated time.

code temperature of maximum time. minutes temperature zone, C.

Flame firing Fibers are heated in a propane-air (propane-oxygen for F3)to the indicated temperature (by an optical pyrometer with no correctionfor A1 0 emissivity) for the time given.

code fiber temperature. C. time, seconds F1 ca 12004400 1 m2 F2 ca1200-1400 F3 1450 about 1 Experience with many fibers indicates that thefired alumina fibers of the examples (all bur Example 11) have mediangrain diameters of less than 3 microns and less than percent of thefiber diameter and a crystallinity of at least 85 percent in view of thespinning compositions and the firing.

It is known (see US. Pat. No. 3,311,481) that fibers made as in Example11 have very fine grain sizes that fulfill the requirements of thisinvention.

The fibers are heated by passing them at a rate of 0.4 ft./minute (0.12meter/minute) through a 26 inch (66 cm.) long furnace wherein thethermal gradient is such that the temperature increases from ambient ateach end to 650C. midway through the furnace. The final stage of heatingemploys a 36-inch (92 cm.) long tube furnace wherein a 6-inch cm.) longzone located midway along the length of the tube is heated to atemperature of 1,500C., both ends of the tube being open TABLE I FiringOxide Particles Oxide Precursor Weight Low High Example Type Parts TypeParts Loss Temperature Temperature 1 A 31.5 Al-l 44.0 1.5% T2 D2 2 A31.7 A1-2 43.6 34.8% T3 D2 3 A 31.6 Al-2 43.7 32.4% Tl D2,D3

CoCl -6H O 0.80 4 A 11.7 Al-2 44.1 32% T2 D5 B 6.7 Cr-l 9.2 KCl 1.4 5(same as 2) 6 A 31.3 Al-l 44.4 4.0% M4 M5,Fl 7 A 32.5 AlCl i6H O 1.54.2% M3 M6,F3

-1 46.2 8 A 30.6 Al-1 43.4 1.9% M1 M7,F2

AlCl -6H O 1.1 9 A 303 A1 1 43.0 0% T1 Fl 10 none Al-S 47.6 32.4% M2 D2,F1

Al-4 8.8 11 none Zr-l 58.5 37.5% T3 D1 Ca-l 5.0 12 none Al-Z 66.7 T3 FlEXAMPLE 1 to the environmental atmosphere. The fibers are This exampleillustrates coated fibers of this invention and the effect of coatingconcentration.

A mixture comprising 1,193.8 g. of an aqueous aluminum oxide dispersion(73.5 weight percent solids), 23.6 cc. of concentrated aqueoushydrochloric acid and 384.4 grams of water is placed in a two litercapacity resin kettle equipped with a helical ribbon stirrer. Thedispersion is simultaneously stirred and heated to 80C., at which time1,223.8 g. of solid aluminum chlorohydroxide, (atomic ratio of aluminumto chlorine 2.01 to 1, equivalent aluminum oxide content =42 percent byweight) are added. The resultant mixture is stirred and heated at 80C.under atmospheric pressure for about 20 hours, during which time thesalt dissolves. The dispersion is then allowed to cool to 26C. withsimultaneous deaeration under reduced pressure, which results in theloss of about 43 cc. of water. Continuous fibers are produced byextruding the deaerated mixture through thirty 0.004 inch (0.01 cm.)diameter by 0.05 inch (0.127 cm.) long spinneret holes using a pressureof 1,585-1815 p.s.i. (1.1 X 10 1.3 X 10 g./cm. The continuous fibers arecollected using a bobbin that rotates at a peripheral speed of 365-450ft. per minute (1 l l-l37 meters/minute).

Following completion of the extrusion, the fibers are cut on the bobbinby a blade moving in a direction parallel to the axis of the bobbin, andthe fibers are collected as a multilayered, substantially unidirectionalarray.

moved through the furnace at a constant rate of speed such that theirresidence time in the 1,500C. zone is two minutes. During this heating,any salts which are present are converted to their respective oxides andthe oxide particles are sintered together to form a unitarypolycrystalline refractory oxide fiber.

A representative sample of the fiber substrate percent alumina) exhibitsa tensile strength of 197,000 p.s.i. (1.38 X 10" g./cm. an elasticmodulus of about 50 X 10 p.s.i. (3.5 X 10 g./cm. a porosity of less thanabout 5 percent, a crystallinity of greater than 85 percent, a mediangrain diameter of about 0.5 micron and a diameter of about 20 microns(0.8 mil.).

The fibers are heated to 1,500C. for two minutes after which they areplaced in a 2-inch (5 cm.) diameter glass tube and subjected to one ormore cycles comprising (1) a five-minute exposure in a moist atmosphereproduced by bubbling nitrogen at a rate of 6 cubic ft./hr. (0.168 meter/hr.) through a water bath at a temperature of 50C.; (2) a five-minuteexposure in a silicon tetrachloride vapor atmosphere produced bybubbling nitrogen at a rate of 6 cubic ft./minute (0.168 meter /hr.)across the surface of liquid silicon tetrachloride at ambienttemperature. The liquid is stirred to increase the rate of evaporation.The coated fiber sample is washed in distilled water for 30 minutes,after which it is dried at C.

The fiber is then heated at 1,500C. for 12 seconds in a tube furnace tovitrify the coating. Analyses and the tensile strength of representativefiber samples are summarized in the following table.

Apparent coating No. of Quantity thickness (cal- Cycles Tensile StrengthElastic Modulus of SiO, culated) p.s.i. (gJcmF) p.s.i. (glcm?) (glm(Microns) 7r of Fiber Diameter 197,000(1.38 10") 50X10 (3.5X10) 0 0 0 l262,000(1.84 10 50 10(3.5X10 0.05 0.025 0.1% 4 306.000(2.l5 10) 0.19 0.10.5% 7 309.000(2.l7X10 0.20 0.1 0.5% 10 264,000(1.85X10 0.71 0.3 1.57113 209,000(1.47Xl0 0.80 0.4 2.0%

Each of the above coated fibers (except the fiber of the l3-cycleexperiment) are characteristic of this invention; the coatings arevitrified, optically uniform layers of the required apparent thickness,adhered to the alumina fiber substrate. The above data show that thecoating of this invention substantially improves the tensile strength ofthe uncoated fiber when even a very thin coating (0.025 micron) ispresent and that no further improvement results at thicknesses aboveabout 0.1 micron. When the 0.3 micron thickness level is reached, it isnoted that about 10 percent of the fibers do not have the coatinguniformity required by this invention. However, the fibers that areproperly coated, as required, do provide a substantial improvement asshown above. At this coating level, the apparent coating thickness of0.3 micron is slightly less than the median grain diameter of firedcoated fibers of this example. (A fiber of the 4-cycle experimentexhibits a median grain diameter of about 0.5 micron.) It is atapproximately this degree of thickness that the zone of spalling isfirst reached as is evident from the nonuniformly coated 10 percent ofthe fibers. The fiber with a 0.4 micron thick coating illustrates aparticular fiber from the batch which does not have the required coatinguniformity of the products of this invention; the tensile strength ofthis fiber is considerably less than the other coated fibers reported.

This effect of the amount of coating on the uniformity of the coating isfurther illustrated for the above fibers by reference to FIG. 2. FIG. 2is a series of photomicrographs obtained utilizing the method (a) fortesting for presence and uniformity of coating. FIG. 2-A is aphotomicrograph of an uncoated sample; this sample does not exhibit thelines, substantially parallel, with respect to the fiber-axis andcoextensive with the fiberoil interface, which would indicate thepresence of a coating. Such lines are seen in FIGS. 2-B, 2-C and 2-D(which are photomicrographs of fiber samples of the above 1, 4, and 7cycle runs); these lines are substantially continuous which indicatesthe required optical uniformity of the coating. FIG. 2-E is aphotomicrograph of two different types of fiber samples of the samel0-cycle run as described above. The fiber in the upper portion of thefigure is satisfactory and is quite similar to that of FIGS. 2-B, 2-Cand 2-D, but the fiber in the lower portion is not satisfactory and isclearly distinct in that the observed line is discontinuous. Thesediscontinuities are believed to represent the points of failure of thecoating (e.g., where the coating has begun to spall ofi or fall away).

307 g. of an aqueous dispersion containing 48.7 percent by weight ofaluminum oxide particles is combined with 415 g. of an aqueous solutionof aluminum chlorohydroxide (Al/Cl 1.84, equivalent to 23.6 percent byweight of aluminum oxide). The resulting mixture is homogenized using adomestic food blender, then heated for 20 minutes at C., after which itis extruded to form continuous fibers at a rate of 450 ft./minute (138meters/minute) using a spinneret with ten holes, 0.004 inch (0.01 cm.)in diameter and 0.050 inch (0.13 cm.) long. A finish comprising a 20/80volume ratio mixture of ethyl laurate in perchloroethylene is applied tothe filaments prior to their being collected on a bobbin. The fibers areremoved from the bobbin as described in Example 1 and heated to 600C.over 45 minutes after which they are allowed to cool to ambienttemperature. The fibers are then fired for 2 minutes at 1,500C. usingthe procedure set forth in Example 1.

A representative sample of the alumina fibers exhibits a tensilestrength of 199,000 p.s.i. (1.40 X 10 g./cm. a modulus of about 50 X 10p.s.i. (3.5 X 10 g./cm. a porosity of less than about 5 percent and adiameter of 19.5 microns.

Individual fibers are heated in a propane-air flame for between 2 and 4seconds after which they are dipped into a 40 Baume aqueous solution ofsodium silicate (Na;Q/L0 1/3.25 which theoretically gives 76 percent byweight SiOZ and 24 percent by weight Na O after firing) and then heatedin a propane-air flame for about 1 second to vitrify the coating. Thetensile strength of a representative coated fiber sample is 287,000p.s.i. (2.02 X 10 g./cm.) and the apparent thickness of the coating is0.55 micron.

The coating procedure is repeated with the starting fiber using anaqueous solution of guanidinium silicate (prepared as disclosed inExample 1 of Yates, U.S. Pat. No. 3,475,375) and lithium chloride (LiO/SiO 1/19) which theoretically gives percent by weight SiO and 5percent by weight Li O, after firing. After heating the coated fibers ina propane-air flame for one second to vitrify the coating, arepresentative sample exhibits a tensile strength of 305,000 p.s.i.(2.14 X 10 g./cm.

EXAMPLE 3 This example demonstrates fibers of this invention utilizingalumina fiber containing various modifiers.

415 g. of an aqueous solution of aluminum chlorohydroxide (equivalent to23.6 percent by weight of aluminum oxide, and exhibiting a Al/Cl ratioof 1.86) is combined with 282 g. of an aqueous dispersion containing 53percent by weight of alumina particles and 3.75 g. of solid cobaltouschloride hexahydrate. The mixture is homogenized using a domestic foodblender, followed by concentration under reduced pressure to obtain a32.4 percent weight loss. The concentrate is then heated for 20 minutesat 80C. under atmospheric pressure. Fibers are obtained by extruding theresulting mixture using a spinneret with nine holes, 0.004 inch (0.01cm.) in diameter and 0.050 inch (0.13 cm.) in length. The fibers arefired as described in the following table.

The foregoing procedure is repeated, with the exception that othermodifiers (nickel chloride, magnesium chloride, cupric chloride,lanthanum nitrate and cadmium chloride) listed in the following tableare used in place of cobaltous chloride hexahydrate.

Individual fibers incorporating each of the modifiers are heated in apropane-air flame for between 2 and 4 seconds, then dipped once into anaqueous solution of guanidinium silicate, and heated for 1 second in apropane-air flame to vitrify the coating. Results are given in Table ll.

The first coated fiber in Table ll has a vitrified coating with anapparent thickness of 0.3 micron.

TABLE II allowed to evaporate. Following a washing with dis tilled waterand drying in air the coating is vitrified by exposing the fiber to atemperature of 1,500C. for 5 seconds. The coating increases fibertensile strength from 207,000 p.s.i. (1.5 X g./cm. to 259,000 p.s.i.(1.8 X 10 g./cm.

EXAMPLE 5 by Weight of Total Oxides Other TENSlLE STRENGTH than AluminaBefore Coating After Coating a 0.5 COO 152,000 p.s.i. 344,000 p.s.i.

(1.07 X 10 g./cm (2.42 X 10" gJcm b 0.5 NiO 220,000 p.s.i. 382,000p.s.i.

(1.55 X 10' g./cm (2.68 X 10 g./cm c 0.05 MgO 189,000 p.s.i. 353,000p.s.i.

(1.33 X 10 g./cm (2.48 X 10" g./cm d 0.5 CuO 194,000 p.s.i. 372.000.s.i.

(1.36 X 10" g./cm (2.61 X 10 g./cm e 1.0 Lap, 172,000 p.s.i. 344,000p.s.i.

(1.21 X 10 g./cm (2.42 X 10 g./cm f 1.0 CdO 211,000 p.s.i. 393,000p.s.i.

(1.48 X 10' g./cm (2.76 X 10 g./cm

EXAMPLE 4 This example illustrates the use of alumina fibers whichcontain silica for the substrate in coated fibers of this invention.

The following materials were combined in a domestic food blender: 76 g.of a 45.4 percent by weight aqueous aluminum oxide dispersion, 66.3 g.of an aqueous dis persion comprising a percent aluminum oxide coatedsilicon dioxide particles [Positive Sol 130M; 87 wt. SiO 13 wt. A1 0manufactured by Du Pont Company];

263 g. aluminum chlorohydroxide (Al/Cl atomic ratio 1.86, equivalent to23.6 wt. percent of aluminum oxide);

27.4 g. of solid chromium chlorohydroxide [Cr (OH) Cl '12l-l O]; and 4.2g. potassium chloride. This mixture is stirred for 5 minutes, thenconcentrated by heating at 80C. under a pressure of 95 mm. of mercury toobtain a weight loss of 32 percent.

A (A1 O /SiO fibe? i s extru dd using a spinneret hole 0.004 inch (0.01cm.) in diameter by 0.050 inch (0.13 cm.) long, and collected on abobbin rotating with a peripheral speed of 700 ft./minute (210 meters/-minute). The fibers are heated to 650C. using the 650C. furnacedescribed in Example 1 and a fiber speed of 0.4 ft./minute (0.12meter/minute), followed by a 1 minute exposure to a temperature ofl,350C.

The fibers which contain 75.9% A1 0 13.2% SiO 8.9% Cr O and 2.0% K 0with a diameter of about 17 microns, are heated for 2 minutes at atemperature of 1,350C., after which they are allowed to remain for 2hours in an atmosphere at 36C. and a relative humidity of 95 percent.The fiber is then dipped momentarily into liquid silicon tetrachlorideand the excess liquid calcium chloride, 23 g. aqueoussolution ofaluminum chlorohydroxide (equivalent to 23.8 percent'by weight A1 0 and15 g. distilled water.

Fiber sampleB is dipped once in a solution compris? ing equal parts byweight of water and weight percent phosphoric acid, and is then dippedonce in a solution comprising equal parts by weight of calcium chlorideand water.

Fiber sample C is dipped once in a mixture comprising 10 g. of anaqueous dispersion of colloidal particles of SiO coated with aluminumoxide (Du Ponts Positive Sol M, containing 26 weight SiO and 4 weight A10 and 30.5 g. of an aqueous dispersion of colloidal aluminum oxideparticles (Alon C) containing 24.2 percent by weight A1 0 The coatedfiber samples are heated in a propane-air flame for about one second tovitrify the coating.

An uncoated fiber sample exhibits a tensile strength of 199,000 p.s.i.(1.4 X 1O g./cm. The coated-samples (A, B and C) exhibit tensilestrengths of 269,000 p.s.i. (1.89 X 10 g./cm. 259,000 p.s.i. (1.82 X 10g./cm. and 232,000 p.s.i. (1.63 X 10 g./cm. respectively.

EXAMPLE 6 This example illustrates a continuous procedure for preparingthe coated fibers of this invention. Fiber Preparation A spin mix ismade by blending an aqueous slurry of A1 0 particulate (60 percentsolids) with solid Chlorhydrol Al (Ol-l) Cl'2l-l O. The spin mix alsocontains a small amount of MgCl -6H O. The mix is transferred to aspinning cell and fibers are continuously spun. The fibers are drawn andaspin finish (20 percent ethyl laurate and 80 percent Percleneperchloroethylene) is applied thereto. The fibers are then wound up,under tension, on a refractory bobbin.

The fibers (on the refractory bobbin) are placed in a muffle furnace andheated to 900C. over a period of 4 hours. The fibers are then placed inanother muffle furnace and heated to l,300C. over a period of 6 hours.At this point the fibers have greater than about 100,000 p.s.i. tensilestrength and can be easily handled without excessive filament breakage.The fibers are then removed from the bobbin anad drawn at the rate of 10ft./min. through a propane-air flame issuing from a 6-inch long ribbonburner. The apparent temperature, measured by an optical pyrometer (withno correction for A1 emissivity at high temperature) is 1,300C.

The resultant fibers (99.5% A1 0 and less than 0.5% MgO) have an averagetensile strength of 227,000 p.s.i. The median grain diameter is 0.47micron.

Fiber Coating and Firing The fibers (in continuous filamentary form) arecontinuously coated and fired by drawing them through the followingzones at 15 ft. (4.55 m.) per minute: (1) Meker Burner (methane-air)flame zone approximately 1.5 inches (3.8 cm.) long; (2) Two inch (5 cm.)steam zone; (3) Three inch (7.6 cm.) long liquid silicon tetrachloridebath; and (4) A flame zone above a 1.5-inch (3.8 cm.) diameter surfaceburner supplied with a propane-air mixture through a 50-mesh per inch(per 2.54 cm.) stainless steel screen for a contact time of about 0.5second apparent fiber temperature is approximately 1,060C., measuredwith a Leeds and Northrup optical pyrometer, Model 8622C. However, noemissivity correction is applied so that the actual temperature is about400 to 600C. or more (depending upon the fiber and coating composition)greater than the indicated temperature.

The resulting fibers are silica coated according to this invention withan average tensile strength of about 264,000 p.s.i. (1.85 X g./cm. Thecoating is g./cm. and is in the form ofa bobbin of yarn (735 denier) of60 continuous filaments.

For coating, the bobbin of yarn is mounted horizontally on a spindle andthe yarn drawn under a freely rotating pulley (polytetrafluoroethylene)submerged in the coating composition in an 8-inch (20 cm.) long bath andthence over 5 jets of nitrogen gas (adjusted to evaporate the bulk ofany solvent diluent in the bath without breaking filaments in the yarn),thence about 0.25-inch (6.4 mm.) above a 15-inch (3.8 cm.) diametersurface burner supplied with a gas'air mixture through a -mesh per inch(per 2.54 cm.) stainless steel screen and wound on a bobbin at 15 feet(4.55 m.) per minute. In some cases the yarn is reheated at the samespeed and omitting the bath which is indicated by multiple yarntemperature values in Table Ill.

Two poly(dimethyl siloxanes) in chloroform are used as the coatingcomposition, A (SF-99 by General Electric Co.) and B (DC-200 by DowCorning Corp.) having 10 and 100 centistokes viscosity at 25C.respectively. Solutions in methyl chloroform are used.

Table III givies the bath composition (volume percent), the temperatureto which the yarn is heated (by optical pyrometer using no emissivitycorrection), tensile strength of the final coated yarns after allindicated heat treatments and apparent coating thickness of the coatedyarns.

All coated fibers in the Table except item g have an optically uniformcoating by method b. Item g has a nonuniform coating apparently as aresult of excessive coating.

A preferred process uses a bobbin of fiber as removed from the hightemperature muffle furnace. The yarn is drawn vertically through anannular propaneoxygen burner and chimney over a finish roll where a 3percent solution of silicone oil (A above) in trichlorethane is appliedand then through a horizontal surface burner and wound up. Appropriatedriving means, guides and tensioning devices are used.

TABLE Ill Apparent Coating ltem Bath Composition Yarn Temp. C. Tensilepsi X 10 psi Thickness ([1,)

(g/cm X 10') a 0.25% A l 105C. 24. 0.01

(1.7) b 0.25% B H25 28. 0.0l5

' (2.0) c 3% B l l 10 29 0.035

(2.0) d 10% B 1085, N00 29 0.05

(2.0) e 20% B H05. lllO 30 .09

(2.1 l' 40% B l I65, H40 30 0.2

(2.l g I00%B I190. lZlO, H80 l3 0.7

about 0.2 g./m., which corresponds to an apparent EXAMPLE 8 coatingthickness of about 0.1 micron.

EXAMPLE 7 This example illustrates a continuous process and the effectof coating thickness.

The alumina substrate fiber employed (containing about 0.2% MgO) has anaverage diameter of 22 microns and a tensile strength of 212,000 psi1.49 X 10 Alumina fibers (containing about 0.24% MgO) with an averagediameter of 48 microns are heated in a propane-air flame for about 3seconds to sinter and straighten them.

The fibers are individually dipped in silicone oil (B of Example 7) andthen held in the propane-air flame for about 1 second to obtain acoating of amorphous vitrified silica with an apparent coating thicknessof 0.25

micron. The coated fibiers have a tensile strength of 192,000 psi (1.35X 10 g./cm. compared to the starting fibers of 121,000 psi (0.85 X 10g./cm.

EXAMPLE 9 The starting alumina fibers (containing about 0.14% MgO) havea diameter of 8.7 microns and a tensile strength of 188,000 psi (1.33 X10 g./cm.

The fibers are dipped once in guanidinium silicate solution of Example 2and fired for about 1 second in a propane-air flame. The coated fibershave a tensile strength of 321,000 psi (2.26 X 10 g./cm.

EXAMPLE 10 The substrate alumina fibers (containing about 1.2% MgO) havea diameter of about 25 microns, a median grain diameter of 0.64 micronand a tensile strength of 140,000 psi (0.98 X 10 g./cm.

individual fibers are dipped 3 times in a colloidal silica dispersion(Ludox 118-40) and then heated for about 2 seconds in the propane-airflame. The fibers with an apparent coating thickness of 0.06 micron havea tensile strength of 258,000 psi (1.82 X 10 g./cm.

EXAMPLE 1 1 This example illustrates the coating of a zirconia fiber.

424.7 g. of a 44% zirconium acetate (H ZrO (OAc) solution in water, 15.8g. of calcium acetate H and 0.8 g. of glacial acetic acid are mixed andplaced in a round bottomed flask attached to a rotary evaporator. Theflask is immersed in a 66C. water bath and rotated slowly while a vacuumof 100-1 10 mm. Hg is applied. Evaporation is continued until 27.8percent of the initial solution weight is removed (about 1 hr.). Foamingis excessive at this point so vacuum evaporation is stopped andevaporation is continued by passing a stream of dry nitrogen over thesurface of the liquid in the rotating flask until a total of 37.5percent of the initial solution weight is lost. The resulting viscous,bubble-filled solution is transferred to a centrifuge bottle and spun ina centrifuge until a clear solution is obtained. This solution has anequivalent oxide content of 35.7 percent. It is extruded through a 0.045in. (0.11 cm.) long circular die orifice having a diameter of 0.004 in.(0.01 cm.). The fiber is allowed to drop through a column at ambienttemperatures into an atmosphere of dry nitrogen and onto a bobbin whichcollects the fiber at a rate of 760 ft./min. (231 m./min.). Prior to thefiber being wound on the bobbin, it passes over a wick saturated with aspin finish comprising a solution of percent by volume ethyl laurate inperchloroethylene. The fibers are removed from the bobbin by cuttingacross them parallel to the bobbin axis. The resulting sheet of fibersis placed in an oven at ambient temperature and raised to 600C. in 45min. The fibers are removed and heated for 1 minute at a temperature of1,500C. in a preheated tube furnace. The tensile strength of the fiberis 87,000 p.s.i.

The substrate fibers (95% ZrO have a tensile strength of 87,000 psi(0.61 X 10 g./cm. and a diameter of 12 microns.

Individual fibers are coated by being dipped 5 times into an aqueousdispersion containing 30 percent by weight of colloidal silicon dioxideparticles (Ludox colloidal silica AS). Following the completion of 5dippings, the fiber is heated in a propane-air flame for 2-5 seconds.The tensile strength is increased to 174,000 p.s.i. (1.22 X 10 g./cm.

EXAMPLE 12 This example demonstrates the use of fibers which have aporosity greater than 20 percent.

180 g. of an aqueous solution of aluminum chlorohydroxide (Al/Cl atomicratio 2; equivalent to 23.8 weight percent of aluminum oxide) isconcentrated to obtain a weight loss of 25 percent. A fiber is extrudedthrough a 0.002 inch (0.005 cm.) diameter spinneret hole. The end of theemerging fiber is secured to a spatula, which is moved away from thespinneret at a rate which yields a continuous fiber that is about 0.3times the diameter of the spinneret hole.

Fibers are heated gradually to 600C. over a 45 minute period, allowed tocool to ambient temperature, and then are placed in propane-air flamefor 1-2 seconds. The fibers (about A1 0 have a porosity of about 30percent.

After being reheated in a propane-air flame for between 2 and 4 seconds,short lengths of fiber are dipped 4 times in aqueous dispersioncontaining 30 percent by weight of colloidal silicon dioxide particles(Ludox AS). After completion of the dipping operation, the coating isvitrified by placing the fiber in a propane-air flame for about 1second.

Coating improves the tensile strength of the fiber from an average valueof 19,500 p.s.i. (0.14 X 10 g./cm. (average of 2 trials) to 63,000p.s.i. (0.44 X 10 g./cm.

The preceding representative examples may be varied within the scope ofthe present total specification disclosure, as understood and practicedby one skilled in the art, to achieve essentially the same results.

The foregoing detailed description has been given for cleamess ofunderstanding only and no unnecessary limitations are to be understoodtherefrom. The invention is not limited to the exact details shown anddescribed for obvious modifications will occur to those skilled in theart.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A high strength polycrystalline refractory oxide fiber having adiameter between about 3 and 250 microns and comprised of grains havinga median grain diameter of less than about 3 microns and less than about10 percent of said fiber diameter,

said fiber having adhered thereto a vitrified coating consistingessentially of a glass-forming oxide, in the form of an opticallyuniform layer, the apparent thickness of said coating being less thanabout 1 micron,

less than about 5 percent of said fiber diameter and less than 9% saidmedian grain diameter.

2. The fiber of claim 1 wherein the refractory oxide fiber is at least60 percent alumina by weight, based on the total weight of said fiber.

3. The fiber of claim 2 wherein the vitrified coating is at least 50%SiO by weight, based on the total weight of said coating.

4. The fiber of claim 3 wherein the apparent thickness of said coatingis between about 0.01 micron and about 0.1 micron.

5. The fiber of claim 4 wherein the porosity of the refractory oxidefiber is less than about 20 percent.

6. The fiber of claim 5 wherein the refractory oxide fiber has aporosity of less than percent, a crystallinity of greater than 85percent, and a grain size distribution wherein substantially none of thegrains is larger than about 3 microns and at least 30 percent by weightare smaller than about 0.5 micron.

7. The fiber of claim 1 wherein said median grain diameter is about 0.5micron.

8. The fiber of claim 1 wherein the vitrified coating is substantially100% SiO 9. The fiber of claim 8 wherein the apparent thickness of saidcoating is between about 0.01 micron and 1 micron.

10. The fiber of claim 9 wherein the refractory oxide fiber issubstantially 100 percent alumina.

11. percent, plurality of the fibers of claim 10 in the form of acontinuous yarn, each refractory oxide fiber having a porosity of lessthan 10 percent, a crystallinity of greater than 85 and a grain sizedistribution wherein substantially none of the grains is larger thanabout 3 microns and at least 30 percent by weight are smaller than about0.5 micron.

12. Process for strengthening a yarn of polycrystalline refractory oxidefibers which comprises:

applying a coating to the surface of each fiber by advancing the yarncontinuously through a fluid composition comprised of a precursor of aglassfonning oxide, said fiber having a diameter between about 3 and 250microns and comprised of grains having a median grain diameter of lessthan about 3 microns and less than about 10 percent of said diameter ofsaid fiber, and advancing the coated yarn to and through a heat zone,thereby heating each fiber to a temperature sufficient to convert saidprecursor to said oxide and to vitrify said coating, the amount ofprecursor applied to the yarn being sutficient to provide said vitrifiedcoating in an apparent thickness of less than about 1 micron and lessthan about 5 percent of said fiber diameter. 13. Process of claim 12wherein said glass-forming oxide is silica, said precursor being appliedin the liquid about 0.1 to 5.0 seconds.

1. A HIGH STRENGTH POLYCRYSTALLINE REFRACTORY OXIDE FIBER HAVING A DIAMETER BETWEEN ABOUT 3 AND 250 MICRONS AND COMPRISED OF GRAINS HAVING A MEDIAN GRAIN DIAMETER OF LESS THAN ABOUT 3 MICRONS AND LESS THAN ABOUT 10 PERCENT OF SAID FIBER DIAMETER, SAID FIBER HAVING ADHERED THERETO A VIRIFIED COATING CONSISTING ESSENTIALLY OF A GLASS-FORMING OXIDE, IN THE FORM OF AN OPTICALLY UNIFORM LAYER, THE APPARENT THICKNESS OF SAID COATING BEING LESS THAN ABOUT 1 MICRON, LESS THAN ABOUT 5 PERCENT OF SAID FIBER DIAMETER AND LESS THAN 1/2 SAID MEDIAN GRAIN DIAMETER.
 2. The fiber of claim 1 wherein the refractory oxide fiber is at least 60 percent alumina by weight, based on the total weight of said fiber.
 3. The fiber of claim 2 wherein the vitrified coating is at least 50% SiO2 by weight, based on the total weight of said coating.
 4. The fiber of claim 3 wherein the apparent thickness of said coating is between about 0.01 micron and about 0.1 micron.
 5. The fiber of claim 4 wherein the porosity of the refractory oxide fiber is less than about 20 percent.
 6. The fiber of claim 5 wherein the refractory oxide fiber has a porosity of less than 10 percent, a crystallinity of greater than 85 percent, and a grain size distribution wherein substantially none of the grains is larger than about 3 microns and at least 30 percent by weight are smaller than about 0.5 micron.
 7. The fiber of claim 1 wherein said median grain diameter is about 0.5 micron.
 8. The fiber of claim 1 wherein the vitrified coating is substantially 100% SiO2.
 9. The fiber of claim 8 wherein the apparent thickness of said coating is between about 0.01 micron and 1 micron.
 10. The fiber of claim 9 wherein the refractory oxide fiber is substantially 100 percent alumina.
 11. percent, plurality of the fibers of claim 10 in the form of a continuous yarn, each refractory oxide fiber having a porosity of less than 10 percent, a crystallinity of greater than 85 %, and a grain size distribution wherein substantially none of the grains is larger than about 3 microns and at least 30 percent by weight are smaller than about 0.5 micron.
 12. Process for strengthening a yarn of polycrystalline refractory oxide fibers which comprises: applying a coating to the surface of each fiber by advancing the yarn continuously through a fluid composition comprised of a precursor of a glass-forming oxide, said fiber having a diameter between about 3 and 250 microns and comprised of grains having a median grain diameter of less than about 3 microns and less than about 10 percent of said diameter of said fiber, and advancing the coated yarn to and through a heat zone, thereby heating each fiber to a temperature sufficient to convert said precursor to said oxide and to vitrify said coating, the amount of precursor applied to the yarn being sufficient to provide said vitrified coating in an apparent thickness of less than about 1 micron and less than about 5 percent of said fiber diameter.
 13. Process of claim 12 wherein said glass-forming oxide is silica, said precursor being applied in the liquid or vapor state.
 14. Process of claim 13 wherein said precursor of silica is silicon tetrachloride.
 15. Process of claim 14 wherein said heating is at a temperature of at least about 1,350*C. for less than about 30 seconds.
 16. Process of claim 15 wherein said heating is at a temperature of between about 1,500* and 1,900*C. for about 0.1 to 5.0 seconds. 